The invention relates to power transmission systems of a type having two or more engine inputs and, more particularly, to split-power transmissions which are arranged to be driven by a high-speed engine, such as a modern turbine engine, and transmit power to a considerably lower speed drive shaft, such as a helicopter rotor drive shaft, wherein the engine power is transmitted through a gear train arrangement incorporating at least one stage in which the power is divided between parallel drive paths.
Conventional power transmissions suitable for use in transmitting power from a high-speed engine to a much slower operating driven mechanism, such as a modern turbine engine driving a helicopter rotor drive shaft, typically utilize four stages of reduction gearing. In order to obtain the required reduction of about 80:1 overall, one of these stages, usually the final stage, comprises a planetary gear arrangement. Planetary arrangements are of high complexity, but predominate in helicopter drive transmissions primarily for the reason that the division of torque between several planet pinions leads to a fairly compact unit at the final stage of the reduction gear. A typical speed ratio up to a value of about 4.7:1 may be achieved in such a planetary gear stage.
A more advantageous helicopter drive transmission is disclosed in U.S. Pat. No. 3,602,068. This patent relates to a split-power transmission in which there are dual-drive paths between the transmission and the helicopter rotor which substantially share equally the load being transmitted from the power source. In a two or more engined installation, the two drive paths from each engine are not combined until the final gear mesh, thus giving drive train redundancy up to this point.
It is also known in the helicopter drive transmission art relating to a plural engined helicopter to provide for at least one pair of parallel-installed engines which are relatively closely laterally spaced and disposed longitudinally aft or forward of the helicopter rotor. Such an arrangement is shown in U.S. Pat. Nos. 3,782,223 and 3,871,248 and Canadian Pat. No. 619,028. Due to the proximity of these helicopter engines with one another, there is considerable risk that a defensive strike to one engine will also destroy or impair the other engine.
It is one of the various objects of the present invention to provide a highly efficient and reliable power drive transmission incorporating gear trains suitable for use in applications requiring a relatively high overall speed reduction, such as in the transmission of power to the main rotor of a helicopter, which is relatively simple in construction and employs a minimum number of reduction stages, gears, and bearings in achieving the overall reduction.
Another object is a helicopter power transmission which allows the engines supplying power to be widely laterally separated and parallel, that is more widely separated than the diameter of the combining gear used to drive the helicopter main rotor.
Another object is to provide a helicopter power transmission in which engine power is divided between parallel drive paths for driving multiple pinions spaced around the rotor combining gear, such that these several pinions are equally loaded. This enables the dimensions of the rotor combining gear to be minimized.
These and other objects and features of the present invention will be apparent from the detailed description of the invention below.